Hmm, I wonder whether this could be related to the site that moves to a new window belonging to a different container. I thought tabs from different containers could share one window (assuming it's not a private window), but maybe that is a configuration option?

I did think of this case. But i have tabs with content from several containers in the same window. And the window this tab reopens in doesn't seem to have more or less tabs of the same container. Sometimes it opens in a window which doesn't have any tabs of that container.

Hmm, I can't think of any built-in features that would do this. (Switch to tab should switch to the open tab matching the URL, not open a new one.) ???

You aren't inadvertently holding down the Shift key (i.e. Shift + Enter) as that would open the link in a new window ?

No, no shift involved.